1. Field of the Invention
The present invention relates generally to fuel assemblies for a nuclear reactor and, more particularly, is concerned with a plurality of water tubes arranged in a cross-like pattern spaced among a plurality of fuel rods in a fuel assembly for providing coolant/moderator water flow paths longitudinally through the assembly without the necessity of altering the standard components and construction of the fuel assembly.
2. Description of the Prior Art
Typically, large amounts of energy are released through nuclear fission in a nuclear reactor with the energy being dissipated as heat in the elongated fuel elements or rods of the reactor. The heat is commonly removed by passing a coolant in heat exchange relation to the fuel rods so that the heat can be extracted from the coolant to perform useful work.
A plurality of the fuel rods are grouped together to form a fuel assembly. A number of such fuel assemblies are typically arranged in a matrix to form a nuclear reactor core capable of a self-sustained, nuclear fission reaction. The core is submersed in a flowing liquid, such as light water, that serves as the coolant for removing heat from the fuel rods and as a neutron moderator. The fuel assemblies are typically grouped in clusters of four with one control rod associated with each four assemblies. The control rod is insertable within the fuel assemblies for controlling the reactivity of the core. Each such cluster of four fuel assemblies surrounding a control rod is commonly referred to as a fuel cell of the reactor core.
A typical fuel assembly in the cluster is ordinarily formed by a N by N array of the elongated fuel rods supported between upper and lower tie plates in laterally spaced-apart relation. An example of such a typical fuel assembly is described and illustrated in U.S. Pat. No. 3,802,995 to Fritz et al wherein a 7 by 7 array of spaced fuel rods is included in the fuel assembly. A problem commonly encountered in a fuel assembly of this type is that the fuel rods in the central region of the assembly may be undermoderated and overenriched. In order to increase the flow of moderator water, Fritz et al substituted one or more elongated empty rods for fuel rods in the central region of the array.
However, the use of one or more water moderator rods in the place of an equal number of fuel rods prevents the use of a full complement of fuel rods in the array with the likely result that a higher linear heat generation rate will be required. The provision of an elongated, centrally-disposed water cross in the form of a cruciform water flow channel such as described and illustrated in the U.S. patent application cross-referenced above sought to remedy the situation. The cruciform water flow channel provides a centrally-disposed, cross-shaped path for the flow of subcooled neutron moderator water within the channel along the lengths of adjacent fuel rods. The water flow channel eliminates the need for water moderator rods and enables use of a full complement of fuel rods.
Notwithstanding the improvements fostered by the cruciform water flow channel of the above-referenced patent application, it too has certain problems. While, the water flow channel provides moderator water in the central area of the fuel assembly, it does divide and iolate the fuel assembly into four separate sections or subgroups of fuel rods. To reduce the possibility of two phase flow instability between the sections, it is necessary to add cross flow holes through the channel. These holes complicate the overall structure of the cruciform water flow channel and result in a rather expensive solution to the problem.
Consequently, the need exists for an alternative approach to the channeling of moderator water to the inner fuel rods of the fuel assembly which is simplier and less costly than the cruciform water flow channel of the referenced patent application.